The present invention relates generally to the field of optical processors, and more particularly to the field of time-integrating optical processors for performing real-time correlations, transforms, and other processing operations.
There are a number of applications where it is desirable to process in real-time, information bearing signals. This is particularly true in the communications and radar processing fields. Normally, in these and similar fields, it is desirable to process in real-time, signals having fairly large information bandwidths. General purpose digital computers are capable of performing some of these processing operations. However, because of their limited speed, they are incapable of performing all but the very simplest of such processing operations in real-time. Special purpose digital signal processors, configured as array processors, typically can perform real-time processing operations if the information bandwidth of the signals is not too large. However, array processors are expensive, sophisticated, hardware devices which are difficult to program, and often the cost of such digital processing at very high data rates is prohibitive.
Because of their large time-bandwidth products and relative simplicity, optical processors represent an attractive alternative to processing large data rate signals. In the past, most optical processors have been of the space-integrating type. The basic principle involved in space-integrating processors is to place one signal into a light modulator so that the time window of the signal containing many cycles is simultaneously present in the optical system. This signal is then made to modulate a light beam to provide an optical signal which contains spatial variations related to the information signal. The resulting optical signal is then imaged with a lens system onto a second signal, which may be displayed in the form of transmission variations in an optical mask (transparency) to provide spatial filtering operations, or the second signal may be introduced as phase variations in the optical signal in a second light modulator. The light modulated by the two signals is then imaged with a second lens onto a single detector whose time-varying output represents the processed input signal. This second lens system integrates the total light signal in spatial dimensions, to provide a signal having intensity variations which is focused onto the single detector. Space-integrating optical processors suffer from the disadvantage that they are limited in time-bandwidth product to the time-bandwidth product of the optical components used in the processor.
Another type of optical processor employs a time-integrating architecture. Time-integrating optical processors basically differ from space-integrating processors in that instead of spatially integrating light onto a single detector, time-integrating devices perform a time integration of the light signal at each point in space. Accordingly, they overcome the limitation of the time-bandwidth product imposed by the optical components employed. Furthermore, they offer a greater flexibility than the space-integrating type of processor, and have less stringent construction tolerances.
A time-integrating correlator is the simplest processing operation to implement using the time-integrating architecture, and is the basic architecture from which other processing operations can be configured. Typical of devices of this type are the time-integrating correlators disclosed in U.S. Pat. No. 3,634,749 to Montgomery, and in Robert A. Sprague and Chris L. Koliopoulos, "Time Integrating Acousto-Optic Correlator," Applied Optics, Vol. 15, No. 1, January 1976. Both references disclose the use of acousto-optic devices as one-dimensional light modulators to provide one-dimensional time-integrating correlators. While these one-dimensional optical processors are useful for performing simple processing operations, there are many applications that require more sophisticated processing which is incapable of being performed using a one-dimensional processor architecture. For example, in the radar processing field, a radar signal is returned from a target shifted both in time and in frequency due to doppler phenomena. This requires ambiguity function processing, to be described more fully hereinafter, which can not be performed by a simple one-dimensional architecture. Such processing requires a two-dimensional architecture. Similarly, there are other processing operations which require a two-dimensional optical processing architecture.
Two-dimensional optical processors may be implemented by utilizing two-dimensional spatial light modulators, such as coherent light valves. Light valves, however, are relatively bulky and expensive devices to use in optical processing systems. Recent advances in optical processing technology, have resulted in significant improvements in acousto-optic devices, such as Bragg cells. These devices are small, compact, and relatively inexpensive. Furthermore, they provide relatively large bandwidths.
Accordingly, it is an object of the invention to provide new and improved two-dimensional optical processors which do not require two-dimensional spatial light modulators.
It is also an object of the invention to provide a time-integrating optical processor architecture.
It is a further object of the invention to provide optical processors capable of performing complex processing operations, such as three-product type processing.
It is a still further object of the invention to provide optical processors employing distributed local oscillators to perform certain processing operations.
It is additionally an object of the invention to provide optical processors employing electronic techniques to provide flexibility and dynamic processing capabilities.
It is also an object of the invention to provide optical processors capable of performing in real-time, processing operations on very high data rate signals.
A time-integrating optical processor having these and other advantages might include, a beam of light, means for modulating the light in first and second mutually orthogonal spatial dimensions using one-dimension spatial light modulators, and a two-dimensional time-integrating detector for detecting the modulated light beam and for providing an output signal representative of the processed information.